1. Field of the Invention
The present invention relates to a controller for an automatic machine comprising a plurality of servo controlled axes, such as an industrial robot (hereinafter, called "robot"), machine tool, or the like, and more particularly, to technology for avoiding dangers caused by malfunction occurring immediately after an automatic device is actuated, and for helping to eliminate the causes of same.
2. Description of the Related Art
In cases where malfunction occurs, for instance, due to deterioration of the electronic components in the servo amplifiers of servo motors driving the various axes of the robot or machine tool, the unit containing the components in question is replaced. When replacing the unit, the connections between the servo amplifiers and servo motors, or between the servo controllers and pulse coders (position detectors), or the like, are temporarily broken and then reconnected once the unit has been replaced. However, in some cases, these connections may be made incorrectly for some of the axes when this reconnection operation is performed.
FIG. 1A illustrates a case where connections are made correctly between devices constituting a servo system for a servo controlled first axis and second axis.
In FIG. 1A, the first axis servo control unit #1 reads in a first axis movement command created inside a controller (illustration of the entire controller omitted), and generates a torque command to be sent to servo amplifier A1, on the basis of the aforementioned movement command and a feedback signal from a pulse coder P1 appended to a first axis servo motor M1. On the basis of the torque command from servo controller #1, the servo amplifier A1 generates a drive current to be supplied to the first servo motor M1. The first axis pulse coder P1 outputs information relating to the position or speed of the first axis to the first axis servo controller #1, in the form of a feedback signal. In the foregoing, the servo system relating to the first axis has been described, and since the servo system for the second axis is the same as that for the first axis, description thereof has been omitted here.
FIG. 1B shows a first example of a case where connections are made incorrectly between devices constituting a servo system for a servo controlled first axis and second axis.
In the example shown in FIG. 1B, the axis correspondences in the connections between the servo controllers and the servo amplifiers have been made incorrectly. If the machine is actuated in this state, the output from the first axis servo controller #1 is transmitted to the second axis servo amplifier A2, and the output from the second axis servo controller #2 is transmitted to the first axis servo amplifier A1.
Consequently, the first axis servo motor M1 operates in accordance with the movement command for the second axis. The pulse coder P1, meanwhile, outputs positional information for the servo motor M1, which operates according to this command, to the servo controller #1. The servo controller #1 generates an output seeking to reduce the positional deviation to zero, but this output is transmitted to the second axis servo amplifier A2, rather than the first axis servo amplifier A1. Therefore, the servo system seeking to reduce the positional deviation to zero will not function and the servo motor M1 will perform completely unexpected and unregulated actions, and depending on circumstances, the probability of runaway operation may be high. There is also a similar risk of runaway operation with regard to the second axis servo motor M2.
Moreover, in a case where, for example, gravitational force or a moment due to gravity is acting on the second axis of a robot, or the like, then even if no command is input, due to the action of gravitational force or the moment due to gravity, an output seeking to reduce the positional deviation to zero will be transmitted to the first axis, and hence there is a risk of the first axis servo motor M1 performing runaway operation.
FIG. 1C shows a second example of a case where connections are made incorrectly between devices constituting a servo system for a servo controlled first axis and second axis.
In the example shown in FIG. 1C, the axis correspondences in the connections between the servo amplifiers and the servo motors have been made incorrectly.
FIG. 1D shows a third example of a case where connections are made incorrectly between devices constituting a servo system for a servo controlled first axis and second axis.
In the example shown in FIG. 1D, the axis correspondences in the connections between the servo motors and the servo controllers have been made incorrectly.
In both the second and third examples where connections have been made incorrectly between devices constituting a servo system for a servo controlled first axis and second axis similarly to the first example, the servo system does not function correctly, and there is danger of servo motors M1 and M2 performing runaway operation.
With regard to the characteristics of a servo system, there is a high probability that the malfunction caused by incorrect connection between the devices constituting a servo system for a plurality of servo controlled axes, as illustrated in the examples in FIG. 1B-FIG. 1D, will arise immediately after the start of the machine. More particularly, in systems incorporating an axis that is subjected to gravitational force or a moment due to gravity, there is a high probability of malfunction occurring immediately after the brakes are released, and in systems which do not incorporate an axis that is subjected to gravitational force or a moment due to gravity, there is a high probability of malfunction occurring immediately after a movement command has been issued.
The reason for this is that since the servo controller generates an output which seeks to make the deviation (difference between command value and feedback value from pulse coder) converge to zero as quickly as possible, then there is a high probability that the system will diverge very rapidly, without achieving convergence, if incorrect connections are made as described above. Moreover, even if there are no incorrect connections as described above and the operator actuates the machine without being aware of the occurrence of deterioration, faults, or the like, in the electronic components, then similar malfunction may be liable to occur.
In order to deal with the problem of malfunctions of this kind, a teaching pendant fitted with a dead-man switch is connected to a controller for a robot, in such a manner that the robot can be brought to an emergency halt by means of the operator releasing (ceasing to depress) the dead-man switch.
However, in a case where the robot runs out of control immediately after the start, for one of the reasons described above, there is a danger of an accident occurring before the operator is able to react. Moreover, in a controller for a machine tool, no means are known for dealing immediately and reliably with a malfunction occurring unexpectedly immediately after the start of the machine tool.